It is well-known in the prior art to measure the characteristics of a semiconductor wafer utilizing a wafer tester assembly as shown in Fig. Reference numeral 101 designates generally a tester body, or a test head, which comprises an internal switching matrix and spring pins 102 which act as testing terminals. Reference numeral 106 designates a probe board comprising a printed circuit wherein the traces on the surface form a signal pattern which is connected to a semiconductor wafer to be tested with test probes or needles 107. Reference numeral 103 designates a personality board which couples test head 101 to the probe board 106 and provides a degree of freedom to the connections between the spring pins 102 and the probe board 106 and prevents damage to the probe board 106 resulting from the direct application of the pressures of the spring pins 102 to the probe board 106. The personality board 103 comprises a printed circuit board having a signal pattern formed or printed on its surface which is connected with the probe board 106 via flexible lead wires 105. The personality board 103 is mounted on a prober by an insert ring 104. Reference numeral 108 designates a semiconductor wafer to be tested. Typically, the geometric shape of the personality board 103 and the probe board 106 is that of an annular disc.
The personality board of the prior art is constructed such that the printed circuit signal patterns are merely enclosed thereon with traces or plated areas of an electrically conducting material known as guard patterns so as to insulate the adjoining signal patterns from each other. The prior art construction fails to ensure complete insulation between the signal patterns thus making it difficult to measure small electric currents, several picoamperes, for example. Solutions to this problem include the development of a personality board having enhanced insulation between the signal patterns effected by separating the signal patterns by slits and enclosing the signal patterns with the guard patterns. This construction is disclosed in Japanese Utility Model Application No. 57-86493.
In the personality board having the latter construction, the leakage current between the signal patterns can be reduced to a remarkably low level thereby allowing the measurement of a very small amount of current. Since, however, the four-wired measuring force lines (for current applications) and sense lines (for voltage detections) are coupled to each other on the personality board, the personality board and the semiconductor wafer must be connected through one wire for each signal. Nor can the four-wired connection be extended to the vicinity of the semiconductor wafer under test DUT. As a result, this prior art personality board is influenced by the impedance of the connecting wires when a high current is applied, such that measurements having acceptable small errors cannot be accomplished.
An objective of the present invention is to provide a personality board which enhances the insulation between signal patterns thereby allowing the measurement of very small amounts of current.
A second objective of the present invention is to provide force lines and sense lines in a separated state to a probe board to effect a four-wired connection in the vicinity of a semiconductor wafer under test so that a measurement with acceptable small errors can be accomplished even when large currents are applied.